Update Go version to 1.21.6

[asf-ci-trafficcontrol]

What does this PR (Pull Request) do?

This PR makes the Go components of Traffic Control build using Go version 1.21.6 and updates the golang.org/x/ dependencies.

See the Go 1.21.6 release notes:

go1.21.6 (released 2024-01-09) includes fixes to the compiler, the runtime, and the crypto/tls, maps, and runtime/pprof packages. See the Go 1.21.6 milestone on our issue tracker for details.

Which Traffic Control components are affected by this PR?

• Traffic Control Cache Config (t3c, formerly ORT)
• Traffic Control Health Client (tc-health-client)
• Traffic Control Client
• Traffic Monitor
• Traffic Ops
• Traffic Stats
• Grove
• CDN in a Box - Enroller
• CI tests for Go components
• Build system - Go version in builder images

What is the best way to verify this PR?

Run unit tests and API tests. Since this is only a patch-level version update, the only changes were bugfixes. Breaking changes would be unexpected.

The following criteria are ALL met by this PR

• Existing tests are sufficient, no additional tests necessary
• The documentation only mentions the major Go version, no documentation updates necessary.
• The changelog already mentions updating to Go 1.21, no additional changelog message necessary.
• This PR DOES NOT FIX A SERIOUS SECURITY VULNERABILITY (see the Apache Software Foundation's security guidelines for details)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (a28af5a) 31.80% compared to head (d0285f9) 28.85%.

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #7901      +/-   ##
============================================
- Coverage     31.80%   28.85%   -2.96%     
  Complexity       98       98              
============================================
  Files           719      602     -117     
  Lines         82819    77445    -5374     
  Branches        970       90     -880     
============================================
- Hits          26340    22344    -3996     
+ Misses        54320    53010    -1310     
+ Partials       2159     2091      -68     

Flag	Coverage Δ
golib_unit	53.84% <ø> (ø)
grove_unit	12.02% <ø> (ø)
t3c_unit	5.88% <ø> (ø)
traffic_monitor_unit	25.47% <ø> (ø)
traffic_ops_integration	69.42% <ø> (ø)
traffic_ops_unit	21.63% <ø> (ø)
traffic_portal_v2	?
traffic_stats_unit	10.78% <ø> (ø)
unit_tests	25.67% <ø> (-3.45%)	⬇️
v3	57.79% <ø> (ø)
v4	79.18% <ø> (ø)
v5	78.58% <ø> (ø)

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